It is proposed that young children's play activities differ in the amount of structure provided to the participating child. Structure is defined as the degree to which there are externally imposed rules or guildelines about appropriate behavior; these can be provided through modeling or through direct instruction and feedback by adults. Activities differing in structure are thought to cultivate different behavioral skills. Low structure activities promote "creating one's own structure" through behavior such as independence, initiative, leadership; high structure activities may promote compliance and dependence on fitting into structures created by others. Differential exposure to high and low structure activities is proposed as one antecedent for sex differences in behavior. It is expected that boys gravitate to low structure activities and girls to high structure activities. Therefore, boys have more opportunity to learn to create structure; girls have more practice in compliance. Three studies are proposed: 1) The first is a correlational study based on naturalistic observation in preschool classrooms. Activity structure is defined by the amount of adult feedback and availability of models. Boys' and girls' participation in different activities and their behavior is observed. Sex differences in participation as a function of structure are expected. Both sexes are expected to show more "creating structure" behavior in low structure activities and more "fitting into structure" behaviors in high structure activities. 2) In the second, activity structure is manipulated experimentally in a series of laboratory sessions, and children's behavior in the laboratory and in the classroom is observed. 3) In the third study, structured or unstructured activities are used as classroom intervention procedures for children whose behavior falls at extremes of passivity or extremes of independence and nonconformity. Participation in high or low structure activities is manipulated experimentally, and changes in classroom behavior are observed.